In the reprocessing of irradiated fuel elements, highly active waste is obtained in the form of highly active liquid concentrates of fission products. These liquid concentrates are solidified by appropriate vitrification processes. Through the addition of glass-forming materials, the radioactive substances are fused into glass in a glass melting furnace. The radioactive glass melt is dispensed from the furnace into so-called steel molds in the form of metal vessels made of high grade steel. After cooling and solidification of the glass block formed and possibly a fairly long period of surface storage, the glass-filled steel molds are then sent to the ultimate waste storage location.
Substantially three methods are known for filling the steel vessel from the glass melting furnace, namely: the bottom discharge system; the overflow system; and, the suction method.
The bottom discharge system basically includes an opening in the bottom of the furnace in which the glass can either be frozen up by cooling or else melted by heating. If the glass in the opening is melted, the glass melt running out fills a steel vessel standing below the furnace.
In the overflow system, the melt is preferably let out via a second chamber of the melting furnace, with a port in the side wall. The second chamber communicates with the main chamber at the bottom of the furnace. When a given filling level is exceeded, the glass runs out of the port in the side wall and through a horizontal overflow pipe into the steel mold.
In the suction method, a partial vacuum is established in the steel vessel and the vessel is sealed in a vacuum-tight manner. After a sealed suction tube mounted on the steel vessel dips into the glass melt from above and after the seal in the suction tube has been melted open, the partial vacuum in the steel vessel causes the glass melt to be drawn by suction into the closed storage vessel.
One difficulty in making the glass block in the steel mold lies in the tendency of the glass to develop fissures. Fissures form in the glass block during the cooling phase. Many different attempts have already been made to minimize this formation of fissures.
A method of minimizing the formation of fissures is disclosed in published German patent application DE-OS 28 46 845 wherein filling elements comprising metal structures are placed in the center of the steel vessel before the glass melt containing the fission products is poured therein. The filling elements may be in various forms and their function is to reduce substantially thermal tensions in the glass block during the cooling phase and to allow a large amount of heat to be conducted away to the wall of the steel vessel.
Results obtained with this method have not been satisfactory. Uncontrolled fissure formations were found to occur in the external part of the glass block.